Extensive metal loss due to corrosion of pipelines can significantly increase the risk of rupture. It is vitally important to estimate the residual strength of corroded pipelines so that proper remedial actions may be taken to avoid catastrophic events. Some prior art methods rely on invasive techniques for measuring the residual strength of corroded structures.
Lacey (U.S. Pat. No. 4,389,877) discloses a system to monitor the amount of erosion taking place within a pipe. A point of reduced strength of the pipe is made by drilling a hole or a notch to a preselected depth in the pipe wall. A hollow casing is provided around the point of reduced strength to provide a sealed zone. A conduit in the hollow casing is connected to a sensing device to monitor pressure changes when pipe failure occurs at the hole or notch.
Others have relied on field simulation. Jones (U.S. Pat. No. 3,846,795) discloses a device in which a housing containing a corrodible member is coupled to the structure so as to expose the corrodible member to the same corrosive environment as the structure itself. The corrodible member is constructed and stressed so as to fail before the structural element. Failure of the corrodible member gives an indication that the failure of the structural element is approaching, allowing shut-down of the system and repair or replacement of failing structural elements.
Currently, the noninvasive assessment of corroded pipelines in the United States is based upon ASME B31G (1991) and modified guidelines RSTRENG (Kiefner and Vieth, 1993). These guidelines are empirically based and are limited in the range of pipe grade and operating conditions for which they yield conservative results. Because they only consider pressure loading, their application precludes failure predictions under combined loadings. Therefore, a reliable analysis model is required to assess the residual strength of a corroded pipe under combined loadings.
Models that apply to pipes subjected to combined pressure, bending and axial loading are rare. Under combined pressure and bending, the existing models usually employ a failure locus in the pressure and moment space. Rupture is presumed to occur if the combination of pressure and moment plots outside of the locus (Smith and Grigory, 1996; Stephens, 1996). This approach implies that under dead loading, bending will reduce the pressure at rupture. However, this need not be the case in the field where displacement controlled bending and axial loading are induced by differential settlement and axial constraint. Recent studies (Grigory and Smith, 1996; Smith et al., 1998; Wang et al., 1998), have demonstrated that the axial stresses within the corroded region due to bending and thermal expansion decrease to nearly zero at rupture provided sufficient strain capacity in the material exists. The implication is that the rupture pressure of a corroded pipe under combined pressure and fixed displacement secondary loading is the same as that for a pipe under internal pressure only, if sufficient strain capacity exists.